Various types of vehicle display devices that illuminate a liquid crystal panel for displaying an image in a vehicle by light emission from a light source are conventionally known. These conventionally known vehicle display devices include those in which the following parameters are variable: the brightness of the light source or the gradation value of the pixels of the liquid crystal panel. (Refer to JP-A-2006-258783, for example.) In these types of vehicle display devices, the visibility of a display image on the liquid crystal panel can be enhanced by taking the following measure: reducing the lightness of the display image by reducing the light emission brightness of the light source or the gradation value of each pixel at nighttime or the like when the outside light intensity is low.
However, the above-mentioned types of display devices for vehicles present a problem. When the light emission brightness of a light source or the gradation value of each pixel is reduced, the lightness of every display image of a liquid crystal panel is reduced. Some of the display images of the liquid crystal panel must be lightly displayed. Even an image requiring some degree of lightness (for example, a warning image providing a warning about any anomaly in the vehicle) is darkly displayed for the above reason. In this case, there is a possibility that the intended purpose of the display of the image is not achieved.